1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly, to an apparatus for transmitting precoding information for an uplink transmission in a wireless communication system supporting multiple base station uplink MIMO and method thereof.
2. Discussion of the Related Art
In the following description, described is a method of transmitting and receiving feedback information in consideration of an uplink channel status of a user equipment.
In a communication system, a receiving stage transmits feedback information on a signal received from a transmitting stage to enable efficient communications. The transmitted feedback information is schematically explained as follows by taking an example for a MIMO communication system.
FIG. 1 is a block diagram of configurations of transmitting and receiving stages of a MIMO communication system according to a related art, in which information fed back from the receiving stage is shown.
Referring to FIG. 1, a scheduler 101 of a receiving stage 100 delivers information for allocating a transmission signal to a proper communication resource using information on a reference signal received from a transmitting stage 200. In this case, the delivered information includes information on MCS (modulation and coding scheme) level, resource and precoding information. Afterwards, the transmission signal is transmitted on a channel via an encoding and mapping 108, an MIMO encoding 109 and an OFDM modulation 110. Meanwhile, the receiving stage 100 receives this transmitted signal and then performs a process reverse to the former process performed by the transmitting stage on the received signal. In particular, the transmitted information is acquired from processes of an OFDM demodulation 102, an MIMO decoding 103, and decoding and demapping 104. Moreover, the receiving stage 100 performs a channel estimation 105 using an OFDM demodulated signal and then acquires feedback information 106 to feed back the estimated channel response to the transmitting stage. The feedback information configures user information 107 and is then fed back to the transmitting stage 200 via the scheduler 101, as shown in FIG. 1.
The information fed back by the receiving stage 100 is classified into OFDM relevant feedback information and MIMO relevant feedback information and is then described as follows.
First of all, based on a measured channel quality information (hereinafter abbreviated CQI) and a channel state information (e.g., SINR (signal to interference and noise ratio), SNR (signal to noise ratio), etc.), a base station determines resource allocation and MCS (modulation and coding scheme) level.
Meanwhile, MIMO OFDM systems can be classified into an open loop (hereinafter abbreviated OL) type and a close loop (hereinafter abbreviated CL) type.
Regarding the codebook based CL-MIMO, a base station estimates a PMI optimal to a current channel status. A modulation order and coding rate vary according to the estimated PMI. Generally, a base station selects a PMI enabling best performance from PMIs applied to the same channel. The base station transmits information on modulation and coding to a user equipment (hereinafter abbreviated UE) together with the selected PMI. The user equipment then performs transmission to be suitable for the received information. The user equipment determines a precoding matrix to apply to an uplink transmission using the PMI information and the like. The user equipment is then able to perform the uplink transmission by applying the determined precoding matrix thereto.
In the MIMO system, it is able to determine the number of ranks applicable according to each channel status. For this, a base station is able to inform a UE of the number of ranks most efficient for the transmission in a channel status of the base station.
As mentioned in the foregoing description, the MIMO system is operable OL or CL. The MIMO system selects to use the OL in fast situation. The MIMO system selects to use the CL in slow situation. For this, the base station selects the OL or CL mode and is then able to inform the UE of the selected mode.
In CL MIMO precoding is applied, it is able to perform single base station precoding in multiple base station environment in a manner that two PMIs (precoding matrix indexes) are combined together in TDD (time division duplex) or FDD (frequency division duplex) uplink transmission to mitigate inter-cell interference.
However, in an uplink MIMO system, when a single base station precoding is performed in a multiple base station environment, how to set a precoding to optimize system performance becomes a problem.